remote sensing Article The MODIS Global Vegetation Fractional Cover Product 2001–2018: Characteristics of Vegetation Fractional Cover in Grasslands and Savanna Woodlands Michael J. Hill 1,2,* and Juan P. Guerschman 1 1 CSIRO Land and Water, Black Mountain, ACT 2601, Australia; [email protected] 2 Department of Earth System Science and Policy, University of North Dakota, Grand Forks, ND 58202, USA * Correspondence: [email protected]; Tel.: +61-262465880 Received: 24 December 2019; Accepted: 24 January 2020; Published: 28 January 2020 Abstract: Vegetation Fractional Cover (VFC) is an important global indicator of land cover change, land use practice and landscape, and ecosystem function. In this study, we present the Global Vegetation Fractional Cover Product (GVFCP) and explore the levels and trends in VFC across World Grassland Type (WGT) Ecoregions considering variation associated with Global Livestock Production Systems (GLPS). Long-term average levels and trends in fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV), and bare soil (FBS) are mapped, and variation among GLPS types within WGT Divisions and Ecoregions is explored. Analysis also focused on the savanna-woodland WGT Formations. Many WGT Divisions showed wide variation in long-term average VFC and trends in VFC across GLPS types. Results showed large areas of many ecoregions experiencing significant positive and negative trends in VFC. East Africa, Patagonia, and the Mitchell Grasslands of Australia exhibited large areas of negative trends in FNPV and positive trends FBS. These trends may reflect interactions between extended drought, heavy livestock utilization, expanded agriculture, and other land use changes. Compared to previous studies, explicit measurement of FNPV revealed interesting additional information about vegetation cover and trends in many ecoregions. The Australian and Global products are available via the GEOGLAM RAPP (Group on Earth Observations Global Agricultural Monitoring Rangeland and Pasture Productivity) website, and the scientific community is encouraged to utilize the data and contribute to improved validation. Keywords: vegetation; grassland; savanna; fractional cover; trend; ecoregion; bare soil; livestock; production systems 1. Introduction Vegetation Fractional Cover (VFC) is an important global indicator of land cover change, land use practice, and landscape and ecosystem function [1,2]. Seasonal dynamics and long-term trends in the fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV), and bare soil (FBS) may identify changes in cropping cycles, impacts of livestock grazing, clearing or planting of woody vegetation, and ecosystem responses to climate shifts. The Australian Vegetation Fractional Cover Product (AVFCP) is derived from the 500 m MODIS (MODerate Resolution Imaging Spectroradiometer) NBAR (Nadir BRDF-adjusted Reflectance) product (MCD43A4) and has been comprehensively documented, validated, and improved over several versions [3–6]. In this study, we present the Global Vegetation Fractional Cover Product (GVFCP) based on the same methodology. Both the Australian and Global products are available through the GEOGLAM RAPP (Group on Remote Sens. 2020, 12, 406; doi:10.3390/rs12030406 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 406 2 of 30 Earth Observations Global Agricultural Monitoring Rangeland and Pasture Productivity) website (https://map.geo-rapp.org). A list of abbreviations appears in Table A1. A key feature of the GVFCP is the explicit sensitivity to FNPV enabling better discrimination of dry cellulosic vegetation and bare soil fractions than is possible with the family of greenness indices. For this reason, the GVFCP is particularly important for monitoring of the non-forested and non-desert global biomes, woodlands, savannas, grassland and shrublands, where the overstory canopy is open, and understory dynamics involve the relationships between cellulosic herbaceous vegetation and/or senescent arboreal leaf litter and bare soil. The first version of the AVFCP [3] used the relationship between the Normalised Difference Vegetation Index (NDVI) and the Short Wave Infrared Ratio (SWIR32; a ratio of the MODIS 2130 nm and MODIS 1640 nm bands) in a linear unmixing method to approximate the relationship between NDVI and the Cellulose Absorption Index (CAI) developed and demonstrated by [7–11]. An improved version of the AVFCP utilizes all seven MODIS NBAR bands plus band transformations and interaction terms in a sophisticated unmixing algorithm [4]. Recent recalibration of the AVFCP with MODIS Collection 6 inputs and an updated validation data set established improvements in the accuracy of the retrieved fractions of photosynthetic vegetation (FPV RMSE 0.112), non-photosynthetic vegetation (FNPV RMSE 0.162), and bare soil (FBS RMSE 0.130) [5]. Based on Australian studies [12–14], the AVFCP provides a consistent and verifiable estimate of the cover fractions for green canopy, senescent vegetation and surface litter, and the visible soil surface. Given the diversity of natural ecosystems across Australia that include tropical rainforest, tropical and temperate savanna, temperate grasslands, semi-arid and arid shrublands and grasslands, temperate eucalypt forests, and temperate rainforest, and the extent of agricultural and pastoral lands across Australia, it is reasonable to propose that the GVFCP should perform with similar levels of uncertainty across the diversity of global land covers and land uses, especially in the non-forested and non-desert biomes. Many global analyses have focused on tree cover change [15–17] and land cover change [18]. The recent comprehensive study of [19] partitions cover into tall vegetation, short vegetation, and bare soil, and the greening analysis of [20] looks at trends in the annual values of the MODIS leaf area index product [21]. In addition, other studies have focused on the total cover and soil or bare fractions [22,23]. There are several global tree cover [24,25] and tree cover change [15] products at multiple resolutions and many FAPAR (Fraction of Absorbed Photosynthetically Active Radiation) [26,27] and green canopy cover or leaf area index products [28–30]. The global cover change study by [19] identifies changes in short vegetation (which includes shrubs under 5 m in height) and bare ground. The cover fractions are mapped from peak growing season canopy [19] meaning that deciduous savanna trees and shrubs are probably fully foliated, and understory grasses are actively growing. By contrast, the GVFCP at full temporal and spatial resolution follows the phenology of overstory and understory throughout the seasons and explicitly retrieves FNPV representing senescent understory. Studies focusing on global scale cover, dynamics, and change in savanna landscapes are less common (e.g., [31,32]), but a study based on persistence of GIMMS (Global Inventory Modeling and Mapping Studies) NDVI signals indicated larger areas of increased than decreased vegetation persistence [33]. However, there has been considerable discussion on the nature of stable states, methods of detection, and drivers of change in savannas (e.g., [34–37]). Due to the specific methodology used to derive it, the GVFCP provides a unique measure of global changes in vegetation fractional cover especially important for woodlands, savannas, grasslands and shrublands which are subject to heavy utilization and conversion for human food production and where cellulosic herbaceous cover is important. It is instructive to provide context for the behaviour of remote sensing products using global datasets that are wholly or partially independent of remote sensing, and describe global vegetation in terms relevant to conservation, ecosystem function, and productivity. The Terrestrial Ecoregions of the World (TEoW; [38]) and the subset of these used to define World Grassland Types (WGT [39]) provides an effective framework within which to view the patterns of VFC for the grassy ecoregions of the world. Remote Sens. 2020, 12, 406 3 of 30 Since these grassy ecoregions are subject to major utilization by humans for extensive and intensive agriculture, the Global Livestock Production Systems (GLPS) classification [40] provides a means to sample variation within WGT Ecoregions due to differences in utilisation between production systems. In this study we have three objectives: (1) To present the GVFCP (using a resampled 5 km2 resolution version) and illustrate the long-term global geographical patterns of FPV, FNPV, and FBS using the TEoW [38]. (2) To document and benchmark the levels and trends in FPV, FNPV, and FB within the WGT Ecoregions at Formation and Division levels [39] using GLPS [40] to provide a measure of internal land use differences. (3) To examine the levels and trends in FPV, FNPV, and FB in savanna, woodland and scrub grassland (SWSG) Ecoregions of the WGT and explore VFC trajectories in selected example ecoregions where major changes are occurring. 2. Materials and Methods The analysis undertaken here provides an overview of global VFC in map form; records benchmark average levels of VFC across production systems for Formation, Division, and Ecoregion areas of the grassy biomes of the world; and documents long-term trends in VFC from 2001–2018 for these areas. The analysis is designed to provide an overview framework and baseline for more detailed future studies. 2.1. Data 2.1.1. Global Vegetation Fractional Cover Product The GVFCP